June 16, 2013

Himasthla elongata

Today's post is bit of a trip down nostalgia lane for me, as the experimental model used in the study we are featuring today is a host-parasite combination similar to one I worked on for somes years during my PhD and postdoc - bivalves and flukes (specifically flukes from a family called the Echinostomatidae - identifiable by their fetching array of collar spines). Much like a parasite that I worked on (Curtuteria australis), Himasthla elongata encysts in the foot muscle of its host and transforms into a stage called the metacercaria (see left photo). But whereas C. australis infects cockles on the mudflats of New Zealand, H. elongata infects mussels on the rock shores of the White Sea.

By embedding itself in the mussel's foot, this parasite hinders the mollusc's ability to move and produce the all-important byssus threads that anchor them to rocks or other substrates. If it becomes infected with too many H. elongata, the mussel loses its ability to use its foot and its survival becomes compromised. Thus this parasite selects for the evolution of mussels that are resistant against it, resulting in a coevolution arms race between the mussels and H. elongata.

To find out how parasites and mussels fare against each other and the role that genetic variants in both the parasite and host population play in coevolution, a group of Russian researchers conducted a series of parasite survival studies and experimental infections. First of all, they did an in vitro experiment where they exposed the infective larval stage of H. elongata (called cercariae) to the blood of different mussels. This was followed by an experimental infection study where they exposed some of those same "blood donor" mussels to H. elongata larvae and measured how well they were they at resisting the parasite.

The researchers obtained parasite-free mussels from an experimental aquaculture farm to act both as blood donors and infection targets for H. elongata cercariae, while the parasites themselves came from infected periwinkles that the researchers collected from an intertidal inlet. These periwinkles harboured the asexual proliferative stages of H. elongata which produce cercariae (see photo on the right). Because H. elongata undergoes asexual multiplication in the periwinkle host, the researchers were able to obtain multiple genetically-identical (clones, essentially) cercariae from each infected snail and test them against a group of genetically-varied mussels.

The researchers paired up 51 different H. elongata clonal lines to blood samples from 161 randomly selected mussels for a total 764 parasite versus host blood combinations* (!). They found that a handful of mussels had blood that killed every single cercaria that came in contact with it and another handful had blood where all the cercariae survived and successfully turn into metacercariae. It seem that H. elongata is adapted specifically to surviving contact with mussel blood (just that it seems that some are better adapted than others), because when they tried to incubate H. elongata cercariae in the blood of the soft-shell clam (Mya arenaria), all the cercariae died within an hour or two.

In a follow-up experiment, they selected 39 of those mussels that had previously served as "blood donors"and exposed each to one of twelve H. elongata clones that were used in the in vitro experiment and found that the results of the in vitro experiment were pretty good indicators of the outcome of those experimental exposures - mussels with blood that killed all the H. elongata they came in contact with were also better than most at fighting off infection by the parasitic fluke. The rest of the mussels were fairly vulnerable to H. elongata and a small handful offered almost no resistance. The larger mussels were generally better at fighting off the parasites with just a little over a quarter of the H. elongata cercariae getting through, while more than half of the cercariae successfully established in the smaller mussels, regardless of the host or parasite genotype.

The parasites themselves also varied in their effectiveness at infecting mussels. Most of the H. elongata clones were fairly good at it, there were a few "superstars" that were especially effective at becoming metacercariae in mussels, while there were also a few "duds" that were hopeless, regardless of which particular mussel they were up against.

What those Russian scientists found with the mussel-Himasthla elongata system does not seem as absolute. Instead, we see variation in overall performance in the population of both host and parasite: there are parasites that ranged from being super effective at what they do, all the way down to complete duds and everything in between. They in turn are going up against mussels with varied level of resistance against them, and how much of a fight those bivalves put up can also be affected by the age and/or body size of the host. However, what it does have in common with those "gene-for-gene"-type coevolutionary systems is that there is a genetic component to either infectivity or resistance, and none of the host are completely resistant to all parasites, just as not all the parasites are completely effective at infecting the available hosts.

Administrators

Why Parasite of the Day? (If it's not every day...)

The United Nations declared 2010 the International Year of Biodiversity. In celebration of the enormous diversity of parasites and to highlight their importance, we created this blog, which showcased a species of parasite every day. Now that 2010 is over, we will continue to add more parasites from time to time, and write about any newly published research on parasite species that we have posted about yet.

See this post from the start of 2011 where we discuss the sheer scale of parasite biodiversity, and this post from the end of 2011 pretty much summarizes the mission of this blog.

Got Parasites?

Got parasites? The American Society of Parasitologists is interested. We invite you to share with us your observations, ideas and questions about parasites. Our members and The Journal of Parasitology represent a wide range of research interests including ecology, evolution, systematics, immunology, biochemistry and molecular biology. Please post any aspect of parasitology you wish to share with us on our Facebook Group Page. Please go to our home page at http://asp.unl.edu/ and look for the ASP on Facebook link.